Light emitting diode (LED) driver circuits are circuits that are used to drive one or more LEDs, typically in a controlled manner. In some instances, LED driver circuits are configured to drive multiple series-connected strings of diodes, known as “LED channels,” but driver circuits that drive single channels, or single diodes, also exist. When driving multiple LED channels, the channels may be operated in parallel with a common voltage node supplying all of the channels. A DC-DC converter (e.g., a boost converter, a buck converter, etc.) may be employed by the LED driver circuit for use in regulating a voltage level associated with the driven LEDs to ensure that all LEDs have adequate operational power. Feedback from the LEDs may be used to control the DC-DC converter. To reduce unnecessary power consumption, the regulated voltage level maintained by the DC-DC converter may be kept to a minimum or near minimum, while still providing adequate power to all LEDs.
During LED driver operation, it may be desirable to vary the light intensity of some or all of the LEDs. One technique for doing this involves driving the LEDs at a variable duty cycle (known as the dimming duty cycle). When a higher duty cycle is applied to the LEDs, a higher light intensity is typically generated. Likewise, when a lower duty cycle is applied to the LEDs, a lower light intensity is generated. Problems may occur, however, when attempting to drive LEDs at dimming duty cycles that are very low. For example, in some systems, a controller associated with a DC-DC converter may be unable to accurately track feedback levels when a dimming duty cycle is too low becalm the LEDs will be “off” for a relatively long time.
In addition, in some cases, a “turn on” time of the LEDs may limit the ability of a driver to support low dimming duty cycles. An LED driver circuit will typically take a finite amount of time to reach a desired LED current level once a drive signal is applied. Any “feedback” provided to the DC-DC converter controller during this “turn on” time can be error prone as the corresponding signal values are in a state of transition. For this reason, feedback blanking is often used to blank out portions of the feedback signal that occur during the “turn on” time. If the “turn on” time of the driver is comparable in duration to then time of the dimming duty cycle (i.e., the time period during which the corresponding LEDs are to be energized), then the DC-DC converter may not have adequate time to properly regulate the target voltage using the available feedback (i.e., the available portion of the feedback signal is not long enough to allow the target voltage to adapt).
Techniques and circuits are needed for improving the ability of LED drivers to operate under short dimming duty cycles.